We love stories that take us to alien planets and let us
explore whole new environments. But not every alien planet is totally
realistic, especially given how much we've learned about exoplanets lately. So
we asked six experts to tell us the biggest mistakes they see in fictional
habitable worlds — and here's what they told us.

Until recently, science fiction creators had to imagine
alien planets, without knowing that much about real-life exoplanets. But in the
past 20 years, we've discovered over 1000 confirmed exoplanets, and learned a
lot more in the process. Here's what our six experts told us bugs them about
alien planets in science fiction:

Not being weird enough

Given the amazing variety of exoplanets we've been finding
out there, and "the remarkable range of orbital and physical
characteristics they seem to have," writers and creators really should be
more bold in the kinds of planets they choose to portray, says Andrew Fraknoi,
chair of the Astronomy Department at Foothill
College in Los Altos Hills, CA.

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"The diversity of exoplanets is apparently so
great that nearly any rendering in science fiction could occur in cosmic
nature," adds Geoff Marcy, Professor of Astronomy at UC Berkeley, who's
credited with discovering 70 out of the first 100 exoplanets we found.

In fact, the best candidates for supporting life at the
moment appear to be tidally locked planets orbiting red dwarfs, says Seth
Shostak, Senior Astronomer with the SETI Institute. "And if you have
biology on planets around such stars, it will be mostly confined to a thin
strip along the terminator. That would be interesting, of course, and quite
unlike most of the planets you see in films."

Assuming exoplanets must have ecosystems as diverse as our
own

People often criticize science fiction for having planets
with a single ecosystem — we've leveled that criticism in the past, in fact.
Science fiction is full of "desert planets" (like Tatooine) or
"ice planets" (like Hoth.) But in fact, there's nothing wrong with
aplanet with a more homogenous
ecosystem than Earth, says Greg Laughlin, Professor and Chair in the Department
of Astrophysics at UC Santa Cruz.

"I've often thought that both Hoth and Tatooine are
quite convincingly presented as representative habitable 'worlds,'" says
Laughlin. "Hoth could credibly be an Earth-like planet in the grip of a
"snowball earth" geologic period, similar to what we know that Earth
has experienced at least twice in its history, and recent research suggests
that desert planets such as Arrakis (Dune) or Tatooine might be better suited
to resist long-term greenhouse warming than a moist planet such as Earth."

"The binary Sun for Tatooine was a brilliant visual
shorthand that immediately telegraphs that the planet is
extraterrestrial," adds Laughlin. If anything, the biggest mistake in
imagining fictional alien worlds might be "to envision planets that
are too similar to Earth. I have a sense that Earth is somehow less
monochromatic than many worlds that present habitable (or marginally habitable)
environments," Laughlin says.

Picking the wrong star

There are two ways that happens, which irritate Stanford University physics professor Bruce A.
Macintosh:

[The first problem is picking] a star that's highly unlikely
to ever host a habitable planet. A good example is Epsilon Eridani. It's got a
good name, very popular choice. But it's also far too young a star to have a
planet that is likely to be habitable - there's good astronomical evidence that
the star is only about 500 million years old - far too young to have really
built up a ecosystem that would build up an oxygen atmosphere. In part because
it's young, the system has a huge number of comets and asteroids (so the planet
is probably getting massively bombarded with meteors), etc. Another very common
example (though less common these days) is to pick a massive hot star (like say
Vega), because massive stars are almost always young (since they have short
lifetimes.)

My second pet peeve is picking a star based on recent and
trendy discoveries - a star which has hints of a habitable planet from current
astronomical measurements. All the possibly-habitable planets we have now are
all marginal and uncertain and the odds that any given one will ultimately
prove to be habitable are very slim.

Not having enough oceans

For a long time, most alien planets "used to all look
like the landscapes around Los Angeles, for obvious reasons," says
Shostak, but at least that's no longer the case. Still, Shostak says one thing
is missing: "How often do you see oceans on these alien worlds? They seem
to be 'all continents,' as far as I can tell."

In fact, Marcy says that continents may be the rarity:

The Earth may be very rare in one attribute few people
realize. The Earth has 0.03% of its mass in ocean water. Yet, the
continents, stemming from plate tectonics, just barely poke above sea level.
Many "Earth-like" planets may receive twice as much water, or even 5 or 10x as
much. Why not? If so, such planets would be more like a bad Kevin Costner
film. Without continents, the development of technology would be slowed and
compromised, if not impossible. How can an intelligent species invent
fire, the printing press, or iPhones on a water world? It seems difficult.

Shoving planets too close together

One thing that bothers Abel Méndez, Associate Professor of
Physics and Astrobiology with University
of Puerto Rico at Arecibo is when you see "multiple
planets or moons close to each other, when viewed from space." It may look
cool, but it's a recipe for planetary collision. "Planets could not be
that close in reality," he warns.

Alien creatures that don't make sense

That could be a whole other article — but Shostak has a few
major gripes about the inhabitants of alien worlds. For example,
"megafauna on planets that look barely able to sustain bacteria."

There's a huge problem with "assuming native complex life
or even intelligent life in desert planets when there are no traces of primary
food source (e.g. plants) to support these life forms," adds Méndez.
"This has problems with how the food chain works and evolution."

Shostak also points to the fact that "intelligent
inhabitants are (1) generally very similar to us in appearance (four
appendages, etc.), and (2) more or less at our level of technical
sophistication, which means we can take them on in dogfights, etc."